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Hello !
ruby 1.9.2p323 (2012-05-22 revision 35755) [x86_64-darwin11.4.0]
Well... I get stuck on this error <https://gist.github.com/3974521>, and
don't know what to do with it...
Thank you for the hand !

Thank you for the reply :-)
The problem occurred after a "bundle update" and we did not check what
got
updated...
The problem comes from
https://github.com/bcardarella/client_side_validations, a plugin that we
used.
Here is the problem : with the ":validate => true", "f" seemed to expect
fields *post-*"bundle update" that it did not check *pre-*"bundle
update".
So i removed ":validate => true" from the following code
<%= form_for([@grade,@tree,@knowledge_cell], :validate => true ) do |f|
%>
...
<% end %>
And it worked...
this solution is more about suppressing the problem that to solve it
properly... Should check the repo !